The present invention relates to organic light emitting diode (OLED) devices, and more particularly, to OLED device structures for improving light output.
Organic light emitting diodes (OLED) are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin film layers of materials coated upon a substrate. However, as is well known, much of the light output from the light-emissive layer in the OLED is absorbed within the device. Because the light emission from the OLED is Lambertian, light is emitted equally in all directions so that some of the light is emitted directly from the device, some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers; see xe2x80x9cModification of polymer light emission by lateral microstructurexe2x80x9d by Safonov et al., Synthetic Metals 116, 2001, pp. 145-148; and xe2x80x9cBragg scattering from periodically microstructured light emitting diodesxe2x80x9d by Lupton et al., Applied Physics Letters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightness enhancement films having diffractive properties and surface and volume diffusers are described in WO0237568 A1 entitled xe2x80x9cBrightness and Contrast Enhancement of Direct View Emissive Displaysxe2x80x9d by Chou et al., published May 10, 2002.
The use of micro-cavities and scattering techniques is also known; for example, see xe2x80x9cSharply directed emission in organic electroluminescent diodes with an optical-microcavity structurexe2x80x9d by Tsutsui et al., Applied Physics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870. However, none of these approaches cause all, or nearly all, of the light produced to be emitted from the device.
Reflective structures surrounding a light emitting area or pixel are described in U.S. Pat. No. 5,834,893 issued Nov. 10, 1998 to Bulovic et al. and describes the use of angled or slanted reflective walls at the edge of each pixel. Similarly, Forrest et al. describe pixels with slanted walls in U.S. Pat. No. 6,091,195 issued Jul. 18, 2000. These approaches use reflectors located at the edges of the light emitting areas. However, considerable light is still lost through absorption of the light as it travels laterally through the layers parallel to the substrate within a single pixel or light emitting area.
There is a need therefore for an improved organic light emitting diode device structure that avoids the problems noted above and improves the efficiency of the device.
The need is met by providing an OLED device that includes a substrate; an OLED having a first electrode formed over the substrate, a layer of organic light emitting material formed over the first electrode, and a second electrode formed over the layer of organic light emitting material to define a light emitting area, wherein the light emitted by the OLED experiences undesirable waveguiding in the device; and a topographical feature located within the light emitting area for disrupting the waveguiding, whereby the light emitting efficiency of the light emitting area is improved.
The present invention has the advantage that it increases the light output from an OLED device.